1. Field of the Invention
The present invention relates to a solid-state image pick-up unit and a method of manufacturing the same, and more particularly to a structure of a gate oxide film.
2. Description of the Related Art
In recent years, in a solid-state image pick-up unit, an increase in the number of imaging pixels to gigapixel or more has progressed and a reduction in a width of an electrode has increasingly been promoted. In such a situation, a gap between the electrodes tends to exceed a resolution limit of photolithography and to be finer in the patterning of an electric charge transfer electrode. Accordingly, there has widely been used a method of forming and patterning a first layer conductive film to form a first layer electrode, then forming an insulating film between the electrodes around the first layer electrode, and furthermore, forming and patterning a second layer conductive film to cover the insulating film between the electrodes.
As shown in an example of a sectional structure in FIGS. 7 and 8, the related-art solid-state image pick-up unit is constituted by providing a photodiode portion and an electric charge transfer portion formed by an electric charge transfer unit (CCD) in a p well (not shown) provided on a surface of a semiconductor substrate 1, and has such a structure that an electric charge generated in the photodiode portion is led to a transfer channel formed by an n-type impurity region (not shown) by an application of a voltage to the electric charge transfer electrode of the electric charge transfer portion and is sequentially read. FIG. 8 is an enlarged view of a substantial part of an electrode portion. More specifically, in the electric charge transfer portion, the electric charge generated in the photodiode portion is led to a transfer channel and is sequentially transferred by applying a voltage to a gate electrode (an electric charge transfer electrode) (labeled 3a and 3b in the figures) to be a reading electrode serving as the electric charge transfer electrode (which will be hereinafter referred to as a reading electrode) which is formed on the transfer channel through a gate oxide film 2 having a three-layer structure including a silicon oxide film (SiO) 2a, a silicon nitride film (SiN) 2b and a silicon oxide film 2c. When a light is incident on the photodiode portion, thus, a photoelectric conversion is carried out in the n-type impurity region so that a signal charge is generated. When a reading pulse is applied to the gate electrode 3 to be the reading electrode serving as the electric charge transfer electrode, the signal charge is moved to the transfer channel. The signal charge generated in the vicinity of the surface of the substrate is accelerated through an electric field generated by the reading pulse and is thus read.
Thus, the gate oxide film formed under the electric charge transfer electrode of the solid-state image pick-up unit has a so-called ONO structure in which a silicon nitride film to be a gate having a high breakdown voltage is interposed between silicon oxide films. With this structure, a thin gate oxide film having a high breakdown voltage is indispensable to a recent solid-state image pick-up unit in which microfabrication has progressed, and the employment of the gate oxide film having the ONO structure (which will be hereinafter referred to as an ONO film) is indispensable for reducing a thickness of a gate film.
In the solid-state image pick-up unit having such a structure, in the related art, the gate oxide film 2 is formed on the surface of the substrate and the electric charge transfer electrode formed by a first layer electrode 3a and a second layer electrode 3b is formed thereon. An insulating film 4 between electrodes which is formed to cover the first layer electrode 3a is constituted by a thermal oxide film, a CVD oxide film or their laminate.
In case of a thermal oxidation, a dense film can be formed. However, it is hard to obtain a sufficient thickness of the film. If a line width is reduced with the microfabrication, moreover, an edge of a polycrystalline silicon film (amorphous silicon) constituting the first layer electrode is easily warped at a thermal oxidizing step and the polycrystalline silicon film to be a conductive film constituting the second layer electrode enters the edge portion in some cases (generation of a stringer which will be described below). In case of a CVD oxide film, a film forming speed is high and a film having sufficiently dense quality cannot be obtained. In further microfabrication of a unit, accordingly, it is hard to obtain a sufficient breakdown voltage with the oxide film and there is a possibility that an electric field might converge on the edge.
Moreover, the gate oxide film has the ONO structure. Therefore, the insulating film between the first layer electrode and the second layer electrode is silicon oxide, while the gate oxide film in a lower layer portion has the ONO structure. In some cases, a film portion of poor quality is formed in the vicinity of a boundary surface of the insulating film between the electrodes which is formed of the silicon oxide and the gate oxide film, and a leakage between the electrodes is caused therein.
In some cases, furthermore, a second layer conductive film for constituting the second layer electrode 3b is provided around the lower part of the first layer electrode 3a and remains as a stringer. For this reason, there is a problem in that the first layer electrode 3a and the second layer electrode 3b in an adjacent cell are connected to each other through the stringer S and a DC short circuit is generated easily. There is a problem in that polycrystalline silicon is removed due to the overetch of the stringer.
Moreover, so-called hydrogen annealing in which a dark current in a photodiode portion is suppressed in order to enhance an initial characteristic and a heat treatment is carried out with the sufficient supply of hydrogen in order to stabilize the characteristic is a very important part for stabilizing the initial characteristic of a unit (JP-A-2003-332556). Therefore, two layers provided on an ONO film over the photodiode, that is, layers provided up to a silicon nitride film are removed to leave a single layer film, thereby forming a passage for the hydrogen. There is employed a method of forming silicon nitride as an antireflection film on the photodiode again after the hydrogen treatment.
On the other hand, a signal charge generated in the vicinity of the surface of the substrate is accelerated with an electric field through a reading pulse. When the signal charge is to be read, a part thereof becomes a hot carrier and is trapped into a silicon nitride film, thereby causing the aging of a reading gate voltage.
In particular, a gate length is reduced when the microfabrication of a unit progresses. For this reason, there is a tendency that the number of collisions of an electron is decreased and the frequency of the generation of the hot carrier is increased. The aging of a voltage to be applied to the reading gate has become serious.
When the microfabrication of a CCD progresses, thus, the ONO film to be the gate oxide film also has a thickness reduced. However, an applied voltage is higher in the CCD as compared with a logic system device. For this reason, there is a problem in that the influence of the hot carrier is great.
In the related-art solid-state image pick-up unit, thus, there is a problem in that a breakdown voltage is not sufficient, that is, an edge convergence on an upper edge of a first layer electrode is caused or a short circuit is caused by a stringer on a lower edge of the first layer electrode with a demand for reducing a thickness of an insulating film between electrodes. In order to implement a gate structure in which a thickness can be reduced with a high breakdown voltage and hydrogen annealing for enhancing an initial characteristic, moreover, it is necessary to subsequently form a hole on a silicon nitride film of a gate oxide film or to once form an opening on the silicon nitride film, thereby carrying out the formation again. In addition, the aging of the hot carrier is not generated in the gate structure. Thus, an enhancement in the quality of the gate oxide film has become a serious problem.